Many experiments have been carried out that demonstrate the feasibility of lasers in solid state gain material which use a single mode waveguide structure (D. C. Hanna in "Tunable Solid State Lasers" edited by M. L. Shand and H. P. Jenssen, published by the Optical Society of America, Washington, D.C., 1989, pages 340 and 350). The waveguides in these cases are either made in the form of a glass fiber or are produced by diffusion of ions into a gain medium, such as, lithium niobate. Both the pump radiation and the laser radiation are confined to propagate in these single-mode guides with little loss in energy except for the absorption of the pump laser radiation. In addition to these single mode waveguide lasers, multimode waveguide lasers have been demonstrated for the case of gas lasers ("High Pressure Waveguide Gas Lasers" P. W. Smith, in Laser Spectroscopy, edited by R. G. Brewer and A. Mooradian, Plenum Press, New York, 1974, pg. 247. "Wideband Waveguide CO2 Laser", R. L. Abrams, ibid). In a usual single mode fiber optical waveguide, the refractive index of the core is greater than the cladding region in order to confine the wave. In the case of multimode waveguide, this need not be the case. The reflectivity of the surface of the interface is a strong function of the angle of incidence and becomes nearly 100% for grazing incidence. The fundamental spatial mode will propagate with the lowest loss in such a structure. Propagation of modes in such waveguides has been described in "Optical Waveguides" by N. S. Kapany and J. J. Burke, Academic Press, New York, 1972, pages 301-319.
Typical dimensions for such a cavity structure are such that D.sup.2 /.lambda.1.ltoreq.1, where D is the width of the cavity resonator, 1 is its length, and .lambda. is the wavelength in the resonator medium. In the case of the carbon dioxide gas laser, the guiding structure has been made from beryllium oxide, aluminum oxide, or copper. For these waveguide gas lasers, the refractive index of the gain medium is near unity.